1. Field of the Invention
The present invention relates to a white ink for an inkjet.
2. Description of the Related Art
Examples of methods used for forming images such as text, pictures or designs on base materials such as films or cardboard include screen printing and gravure printing, but in recent years, inkjet printing, which is effectively a plateless printing technique, has been attracting considerable attention.
When printing to the types of base materials mentioned above, the underlying substrate must be covered to enable better color development within the printed item. A white ink having favorable covering properties is generally used to cover the substrate, and an inorganic pigment, in particular titanium dioxide, is often used as the pigment for this white ink. However, because inorganic pigments such as titanium dioxide have a high specific gravity, suppressing pigment sedimentation tends to be a problem when such pigments are used in low-viscosity inkjet inks.
If the particle size of the titanium dioxide is reduced in order to suppress this pigment sedimentation, then although the level of sedimentation can be reduced, a problem arises in that the white coloring properties tend to deteriorate, resulting in inferior covering properties and a dramatic deterioration in the hiding power of the white ink.
Patent Document 1 (Japanese Patent Laid-Open No. 2004-339388) discloses an aqueous white ink composition that can be used in flexo printing or gravure printing, wherein by using a pigment component composed of titanium dioxide, silica and another white inorganic pigment, a favorable degree of whiteness can be maintained while ensuring good storage stability. However, the pigment component comprises titanium dioxide as the main constituent, and because the disclosed ink is a comparatively high-viscosity ink, if the composition was applied to the preparation of an inkjet ink, then maintaining favorable storage stability would be problematic.